How to dress

This time of year can be so confusing! Technically, it's fall, but the weather can't seem to decide. It's chilly and cloudy one day then sunny and warmer the next day. My little one is 18 months old, is it still okay for me to let her wear some of her summer clothes that are sleeveless? Or, should I try to keep her in pants more often? I don't worry about it too much on the days we stay home, but I worry when we go out and about.